By default it create a key with strength of 2048 bits.
The two key files are:~/.ssh/id_rsa this is the private key~/.ssh/id_rsa.pub this is the public key, you upload this key to the server you want to connect with ssj.

if we want key stronger than 2048 bits, we can use the -b option.
4096 bits is usually supported by most server, we create key with this strength by running the following:

ssh-keygen -b 4096

ssh-keygen -pwe can use the -p option to remove or change the passphrase of the key file, as long as we know the passphrase.